The cell is a dynamic structure that is constantly
trying to keep up with the changes in its environment. Most cells go
through a series of changes in order to maintain
homeostasis. One
of these changes deals with reproduction of the cell. Cells need to
reproduce when their surface area can no longer supply their much
larger volume with nutrients and rid itself of metabolic wastes. The
first part of the cell cycle is called Interphase. Interphase
is considered the resting stage of the cell. This definition can be
misleading since the cell is not really resting. During interphase
the cell is carrying out its everyday activities. This part of
interphase is called the G1 phase. It may last
several days or several years depending on the type of cell involved.
When the surface area to volume imbalance occurs the S phase
begins. Here the genetic material replicates itself. This allows the
cell to contain enough material to fully supply 2 cells upon
division. The G2 phase soon begins. Here the
cellular organelles are produced to allow for an adequate amount for
the new cell being produced. After interphase Mitosis begins.
Mitosis is considered nuclear division, since the focus is upon the
genetic material of the cell. The first part of mitosis is
Prophase. During prophase the nucleus begins to disappear. The
chromatin pulls together and forms pairs of rope-like structures
calledchromatid
pairs. These
chromatid pairs are actually identical chromosomes that developed
during the S phase of interphase. The chromatid pairs are held
together by a structure called the centromere. Spindle
fibers also begin to appear and along with the
centrioles;they
migrate to the poles of the cell. Metaphase begins when the
chromatid pairs line up along the center of the cell. This makes it
possible for the chromatids to position themselves so that they can
migrate to the opposite poles of the cell. Anaphase is the
stage where this process occurs. The chromatid pairs split and the
spindle fibers contract pulling each chromosome toward their pole.
This process continues until the chromosomes arrive at each pole. The
nucleus reappears and the spindle fibers disappear. Telophase
begins at this point and will continue until the cell splits into
two. This split is called cytokinesis. We now have 2 daughter
cells, each identical in the number and type of chromosomes. They are
smaller than the mother cell and will begin to develop starting
interphase again.

Prophase

Metaphase

Anaphase

Telophase

Meiosis

Gametes are special cells used in sexual
reproduction. It is important that these cells contain 1/2 the normal
number of chromosomes than the regular body cells (autosomes). When
the sperm and egg unite the resulting zygote will contain the normal
number of chromosomes for that organism. A special process called
meiosis occurs in the reproductive organs to allow this to occur.
Meiosis is a bit more complex than the cell cycle mentioned above;
although many of the terms and stages show many
similarities.

Before meiosis occurs the cell duplicates its
chromosomes.

Interphase

Prophase I : The nucleus disappears,
spindle fibers form , and the chromatid pairs form. A major
difference occurs here separating this prophase from mitosis
prophase. The chromatid pairs pair up with their homologous pair,
forming a tetrad. This tetrad contains 2 original homologous
chromosomes and their duplicates. The process of crossing over occurs
during this time allowing for genetic diversity in the genome.
Crossing over is a process where pieces
of two homologous chromosomes, switch places and end up on the other
chromosome. (1 cell )

Metaphase I : Tetrads line up along the
center of the cell. (1 cell )

Anaphase I : The tertads split and move in
opposite directions. During this phase the chromatid pairs move away
from each other. (1 cell )

Telophase I : Two cells form each
containing a complete set of chromosomes in the form of chromatid
pairs. (2 cells )